Zinc Oxide-Catalyzed Dehydrogenation of Primary Alcohols into Carboxylic Acids

Zinc oxide has been developed as a catalyst for the dehydrogenation of primary alcohols into carboxylic acids and hydrogen gas. The reaction is performed in mesitylene solution in the presence of potassium hydroxide, followed by workup with hydrochloric acid. The transformation can be applied to both benzylic and aliphatic primary alcohols and the catalytically active species was shown to be a homogeneous compound by a hot filtration test. Dialkylzinc and strongly basic zinc salts also catalyze the dehydrogenation with similar results. The mechanism is believed to involve the formation of a zinc alkoxide which degrades into the aldehyde and a zinc hydride. The latter reacts with the alcohol to form hydrogen gas and regenerate the zinc alkoxide. The degradation of a zinc alkoxide into the aldehyde upon heating was confirmed experimentally. The aldehyde can then undergo a Cannizzaro reaction or a Tishchenko reaction, which in the presence of hydroxide leads to the carboxylic acid.